CODEWITHSUNDEEP
swiftswiftuinsnotificationcenter
Vladislav
Vladislav

Reputation: 1502

Why I can't receive a notification on an observer class inside a View struct in SwiftUI?

I have the following code, where I create a class that acts as an observer of a default notification. But the notification never arrives:

struct NotificationCenterExampleView: View {
    
    let observerA = ObserverClassA()
    
    init() {
        print("NotificationCenterExampleView init")
        
        
        NotificationCenter.default.addObserver(observerA, selector: #selector(ObserverClassA.receivedNotification(notification:)), name: Notification.Name("CustomNotification"), object: "This is the message")
        let notificationToPost = Notification(name: Notification.Name("CustomNotification"), object: "Message being sent", userInfo: nil)
        NotificationCenter.default.post(notificationToPost)
    }
    
    var body: some View {
        Text("Notification Center Example")
            .frame(minWidth: 250, maxWidth: 500, minHeight: 250, maxHeight: 500)
    }
}

class ObserverClassA: NSObject {
    
    @objc func receivedNotification(notification: Notification) {
        let message = notification.object as! String
        print("Message received: \(message)")
    }
}

I know that using .publisher and onReceive this will work inside the View struct, but what's the actual reason for this code to not work?

Upvotes: 2

Views: 727

Answers (1)

Asperi
Asperi

Reputation: 257749

Notifications matched by object, so if you subscribe by one object but post with another object, then subscriber is not fired.

Here is fixed variant. Tested with Xcode 12.1.

    NotificationCenter.default.addObserver(observerA, 
       selector: #selector(ObserverClassA.receivedNotification(notification:)), 
       name: Notification.Name("CustomNotification"), 
       object: nil)   // << subscribe for all

    let notificationToPost = Notification(name: Notification.Name("CustomNotification"), object: "This is the message", userInfo: nil)
    NotificationCenter.default.post(notificationToPost)

Upvotes: 1

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